1. Field of the Invention
The present invention relates to a method for the production of a glass suitable for an information recording medium that is required to have high qualities, a process for the production of a glass substrate blank and a glass substrate from the glass obtained by the above production method, an information recording medium to which the above glass substrate is applied, and a glass melting apparatus for use in the above glass production method.
2. Prior Art of the Invention
Generally, as a material for a glass-contact-portion of a glass melting apparatus (to be sometimes referred to as “furnace material” hereinafter), an AZS (Al2O3—ZrO2—SiO2-containing) electrocast refractory material is used for a high-temperature portion and an alumina-containing electrocast refractory material is used for a relatively low-temperature portion. These furnace materials are desired for reasons that they cause less contamination to a glass and causes foaming to a less degree. For example, these materials are used in a glass-contact-portion of a melting apparatus for soda lime glass, a glass for CRT, an optical glass and a glass for electronic devices. For the same reason, it is considered desirable to use an AZS electrocast refractory material as a furnace material in a glass-contact portion for a glass as a material for a substrate of an information recording medium such as a glass substrate for a magnetic disk.
Meanwhile, it has been mainstream practice to use an aluminum substrate as a substrate for a conventional magnetic disk. With increasing demands for a higher density of a magnetic disk, the substrate material for the magnetic disk is shifting from an aluminum material to a glass material since the glass material has a high Young's modulus and can easily accomplish the flatness of a disk surface. Further, with an increase in the density of a magnetic disk, the distance between a recording medium and a write/read head decreases markedly. These days, the above distance is as close as a distance of approximately a few molecules arranged side by side. In operation of a magnetic disk, it is required to turn the disk several thousand times a second while maintaining the above fine distance as a distance between the head and the disk. The above state is something like a state in which a large jetliner flies tens centimeters above a ground without colliding with any obstacle, and the fact is that the surface of a substrate for an information recording medium is required to be free of any fine projection.
One of the substrate surface defects that hamper the above higher-density recording is a ZrO2 mound problem. It is assumed that the ZrO2 mound is caused as follows. ZrO2 has a higher hardness than glass. When the surface of a glass substrate containing ZrO2 particles is polished, the ZrO2 particles present in the surface decrease slowly and the glass decreases rapidly. As a result, ZrO2 particles present in the surface come to appear as fine projections on the substrate surface. These fine projections are zirconia mounds, and when the projections are present, forms of such projections are reflected on the surface of a recording medium and the projections cause a head crash that is a collision of a head with the surface of the magnetic disk. In high-density recording media provided these days, even the presence of only one fine ZrO2 mound causes the recording media to be defective. When a glass as a material for a substrate for an information recording medium is melted, therefore, it is required to perfectly prevent the inclusion of ZrO2 particles.
There are large glass demands in the above field, and under the circumstances, such demands can be fulfilled only when glass is mass-produced. There is therefore required a technique for stably producing a large amount of a glass containing no ZrO2 particles.
When a glass as a material for a substrate of the above information recording medium is melted in a glass melting apparatus made of an AZS electrocast refractory material, the ZrO2 mound problem is liable to be caused. It has been considered that the above problem is caused as follows. When the ZrO2 content in a glass is large, for example, 5% by weight or more, ZrO2 cannot be melted well, so that a non-melted substance of ZrO2 is liable to be generated. Therefore, attention has been paid only to the content of ZrO2, and no attention has been paid to the relationship between a melting apparatus and the generation of a ZrO2 crystal. Under the circumstances, it has been impossible to obtain a large amount of glass substrates that contain no ZrO2 crystal grains and are free of ZrO2 mounds.